MGO-CODOPING-INDUCED CHANGE IN THE SITE DISTRIBUTION AND VALENCE STATE OF CHROMIUM IONS IN LINBO3

The optical and electron spin resonance (ESR) spectroscopies of LiNbO3 :Cr(0.1 at%) and codoped LiNbO3:Cr (0.1 at%), Mg(X at%) crystals (X = 2 to 6) have been investigated. In undoped, 2 at% and 4 at% MgO-doped samples, the optical absorption and ESR spectra are attributed to Cr3 ions in Li+ sites. In crystals with MgO-doping levels greater than 4 at%, Cr3+ ions located in Li+ and Nb5+ sites were observed too. A new emission band centred at longer wavelength is due to the T-4(2) --> (4 )A(2) transition associated to Cr3+ ion and discussed assuming chromiu m valence change.